1. Field of the Invention
The present invention relates to a gas laser oscillator adopting a method for cooling a laser gas by a heat exchanger, in particular, a gas laser oscillator capable of controlling a temperature of a resonator unit.
2. Description of the Related Art
Laser processing machines have been known which perform machining, such as cutting and punching, by irradiating metal and resin materials with laser beams. Such laser processing machines are often equipped with carbon dioxide laser oscillators which can generate large output.
A carbon dioxide laser oscillator comprises a resonator unit which excites a gas mixture (hereinafter, referred to as a laser gas) mainly consisting of carbon dioxide, nitrogen, and helium and resonates a beam generated from the excited laser gas to amplify the beam. The resonator unit includes a discharge tube for storing the laser gas and a total reflection mirror and a partial reflection mirror (an output coupler) disposed on both ends in a long axis direction of the discharge tube. When the laser gas in the discharge tube is excited by a discharge, a beam is generated in the long axis direction of the discharge tube. The generated beam is amplified by being repeatedly reflected between the total reflection mirror and the partial reflection mirror (the output coupler) and output to the outside of the resonator unit by transmitting through the output coupler.
When the laser gas in the discharge tube is excited by a discharge to generate a laser beam, 10% or more of a discharge energy applied to the laser gas is converted to the beam. Remaining discharge energy generates heat and increases a temperature of the laser gas. Accordingly, a temperature of the resonator unit is increased which may deform components constituting the resonator unit. Thus, in the gas laser oscillator, the laser gas of which temperature becomes high is drained out from the discharge tube, cooled by a heat exchanger, and returned into the discharge tube. In addition, cooling water is circulated through the heat exchanger and controlled at a constant temperature by a cooling device, such as a chiller, on the outside of the laser oscillator. Temperature increase of the laser gas is thus suppressed, so that the components constituting the resonator unit are less likely to be deformed, and a laser output can be stable.
In particular, the output coupler and the total reflection mirror which are the components constituting the resonator unit are positioned with high accuracy, and the temperature control of the resonator unit is an important technique for stabilizing the laser output. Thus, many techniques have been discussed for controlling temperatures of resonator units.
Japanese Laid-open Patent Publication No. 2001-57452 discloses a gas laser oscillator which comprises a flow path for circulating a cooling medium in an optical bench holding reflection mirrors of a resonator and an electromagnetic valve for opening and closing the flow path. The gas laser oscillator measures a temperature of the cooling medium flowing through the optical bench by a temperature sensor and opens and closes the electromagnetic valve depending on a measured value. Accordingly, temperature increase of the cooling medium is suppressed, and a temperature of the resonator unit is maintained within a predetermined temperature range.
Japanese Laid-open Patent Publication No. H01-232779 discloses a gas laser oscillator including a mirror cooling system for circulating cooling water around mirrors of a resonator unit, a heat exchanger for cooling a laser medium, and a laser medium cooling system for circulating the cooling water through the heat exchanger. The gas laser oscillator further comprises piping connecting the laser medium cooling system and the mirror cooling system and an electromagnetic valve for opening and closing the piping. When the temperature of the cooling water in the mirror cooling system is lower than a predetermined temperature, the electromagnetic valve is closed. When the temperature of the cooling water in the mirror cooling system is higher than the predetermined temperature, the electromagnetic valve is opened, and a part of the cooling water circulating through the heat exchanger of the laser medium cooling system is supplied to the mirror cooling system.
Japanese Laid-open Patent Publication No. 2001-24257 discloses a method for equalizing temperature distribution of a resonator unit by circulating water controlled at a constant temperature spirally around the resonator unit.
Japanese Laid-open Patent Publication No. 2009-117700 discloses a gas laser oscillator including a gas flow path for sealing a laser gas in the gas laser oscillator and a heat exchanger for cooling the laser gas in the gas flow path. The gas laser oscillator further comprises, in the gas flow path, a gas detour for detouring around the heat exchanger and a gas flow rate regulating valve for regulating a flow rate of the laser gas flowing through the gas detour. The gas flow rate regulating valve regulates the flow rate of the laser gas passing through the heat exchanger, and thus the temperature of the laser gas is controlled.
The above-described resonator unit resonates the laser beam by positioning the respective reflection mirrors to be disposed on the both ends of the discharge tube with high accuracy. Therefore, if a position of each reflection mirror is shifted, an operation of the carbon dioxide laser oscillator may become unstable, such as occurrence of reduction of a laser output.
In a positioning operation of each reflection mirror, the carbon dioxide laser oscillator is operated while adjusting the position of each reflection mirror, and each reflection mirror is fixed at a position at which a predetermined output is stably oscillated, namely a position at which a steady state is obtained. In other words, each reflection mirror is placed on a position at which the resonator unit obtains a temperature distribution of the steady state. Thus, immediately after a start-up of the carbon dioxide laser oscillator, the temperature distribution of the resonator unit is different from the temperature distribution in the steady state in some cases. In this case, the position of each reflection mirror may be shifted from the position in the steady state, and the operation of the carbon dioxide laser oscillator may be unstable. In particular, when the carbon dioxide laser oscillator is used in a cold region or in a winter season, a problem notably occurs that an operation of the carbon dioxide laser oscillator immediately after a start-up becomes unstable.
As a countermeasure against the above-described problem, the temperature of the resonator unit is increased by starting up the carbon dioxide laser oscillator and performing a gas discharge in the discharge tube for a certain period of time. When the temperature distribution of the resonator unit becomes that in the steady state, it is regarded that preparation for laser oscillation is complete, and laser oscillation from the resonator unit is permitted. However, such a countermeasure has a problem that it takes a time from the start-up of the carbon dioxide laser oscillator to the completion of the laser oscillation preparation.
In addition, the carbon dioxide laser oscillator is designed to radiate heat generated by the resonator unit to the outside. Specifically, when the carbon dioxide laser oscillator is started up, cooling water controlled at a constant temperature by the chiller is circulated through the heat exchanger so as to cool the laser gas by the heat exchanger. Therefore, when the carbon dioxide laser oscillator is started up from a cold state, the laser gas is cooled, and it hinders the temperature increase of the resonator unit by contrast. Accordingly, there is also a problem that a period from the start-up of the carbon dioxide laser oscillator to when an oscillation operation can be stably performed becomes longer.
Any of Japanese Laid-open Patent Publications No. 2001-57452, No. H01-232779, No. 2001-24257 and No. 2009-117700 disclose only techniques for cooling the resonator units including the laser gas and the reflection mirrors so that temperatures of the resonator units do not exceed predetermined temperatures. In other words, the gas laser oscillators described in Japanese Laid-open Patent Publications No. 2001-57452, No. H01-232779, No. 2001-24257, and No. 2009-117700 only perform control to suppress temperature increase of the resonator units after start-up of the gas laser oscillators. Therefore, these techniques have problem that they take times from the start-up of the gas laser oscillators to when oscillation operations can be stably performed when the gas laser oscillators are used in cold regions or in winter seasons.